1. Field of the Invention
The present invention relates to an electrophotographic developer, an electrophotographic developing method and a process cartridge for electrophotography.
2. Discussion of the Background
In electrophotography, an electrostatic latent image formed on a photosensitive medium is developed by a developer. One-component developers composed of a toner and two-component developers composed of a toner and a carrier, such as glass beads and magnetic particles with or without resin coating, are known as the developer. Two-component developers are suitably used for high speed printing and copying machines. In digital electrophotography in which a photoconductor is irradiated with a laser beam to form an electrostatic latent image, two-component developers are generally used for developing the latent image.
Recently, there has been an increasing demand for a developer which can meet the requirements of high resolution, improved reproducibility, high image density and of multi-color images. For example, minimization of the smallest unit of latent images and increase in image density of the latent image are desired. Accordingly, there is a great demand for a developer which can accurately and precisely develop a dot image having such properties. To meet this demand, various proposals have been made from the standpoint of both, process conditions of the developing process and the developers (toners and carriers).
As to process conditions, minimization of the developing gap, use of a thin film photoconductor and reduction of the beam diameter for writing are considered to be effective. However, these measures increase cost and reduce reliability.
As for developers, the use of a toner having a small particle size will greatly improve the reproducibility of dot images. However, background stains and reduction of color density occur. Additionally, when a toner having a small particle size and a low softening point resin is used for full color image formation, significant adhesion of the toner particles on the surface of the carrier occurs. The developer deteriorates during use causing toner dispersion and background stains.
The use of a carrier having a small particles size has been suggested. For example, in Japanese Laid-Open Patent application JP 58-144839, it was suggested to use a magnetic carrier having an average particle diameter of less than 30 μm and comprising ferrite particles having a spinel conformation. However, the carrier is not resin coated and is useful for developing in a low electric field. The carrier has inferior developing ability, and has a short life time due to the lack of a resin coating.
In addition, Japanese patent JP 30-29180 discloses a carrier for electro-photography having: a 50% average particle diameter (D50) of 15-45 μm, 1-20% of carrier particles smaller than 22 μm, less than 3% of carrier particles smaller than 16 μm, 2-15% of carrier particles larger than 62 μm, less than 2% of carrier particles larger than 88 μm, the carrier particles satisfying the condition of equation (2) wherein S1 is the specific surface area measured by an air permeability method, and S2 is the specific surface area calculated by equation (1).S2=(6/ρ·D50)×104  (equation 1)
(wherein ρ is the specific gravity of the carrier)1.2≦S1/S2≦2.0  (equation 2).
When a carrier having a small particle diameter is used, the following advantages are obtained.
[1] Because the surface area per volume is large, such carrier can give enough frictional charging for an individual toner. Therefore, there is little static build-up and reverse static build-up in the toner, and little background fouling occurs. In addition, there is little adhesion and smearing of the toner around a dot, little blurring, and the dot reproducibility is good.
[2] Due to the large surface area per volume and low background fouling, the charging quantity of the toner levels off and an image having a good density can be provided.
[3] Because the carrier has a small particle diameter, a minute magnetic brush can be formed. In addition, because of good flow properties of the tip of the brush, the image has little traces from the tip of the brush.
However, conventional small diameter carriers have a drawback because the carrier particles adhere to the photoreceptor surface, and the adhered carrier causes streaking and fuser roller streaks on the photoconductor. In particular, if the average particle diameter is smaller than 32 μm and the particle size distribution is broad, carrier adhesion occurs, which is disadvantageous.
For stable picture quality, it is important to reduce the dispersion of the charging quantity (charging quantity distribution) of the developer. The charging quantity of the developer is correlated with the size of the toner and the carrier particle size. Therefore, it is possible to reduce the dispersion of the charging quantity by reducing the dispersion of the particle size. This results in a narrow charging quantity distribution.
Japan-Laid Open patent application JP 2002-207309 discloses a toner having a proportion Dw/Dn=1.00-1.20 and a weight average particle diameter Dw=2-6 μm. Using a narrow particle size distribution, the intermolecular force between the electrostatic latent image and the photoconductor becomes constant and the transferal characteristics are improved. Using a small average particle diameter, the quantity of electro static charge of the toner is increased. If the particle size distribution is narrow and the width of the amount of triboelectric distribution is broad, the gray scalability improves. However, if the quantity of the electro static charge of the particle is in inverse proportion to one or two multiples of the particle size and if average particle diameter is decreased, and if the amount of triboelectric distribution is sharp, an improvement of the gray scalability cannot be expected.
Judging from standpoint of stabilization of picture quality, a sharp width of the amount of triboelectric distribution of the developer is preferable. However, the triboelectric distribution amount of the developer is depending on not only the toner but also particle size distribution of the carrier. Therefore, the prescription of the particle size distribution of the toner and the average particle diameter of the toner are not enough to control the triboelectric distribution of developer.